Electronic cigarettes are the most used tobacco product among youth, and are used by over 30% of adult tobacco smokers (dual users). Electronic cigarette aerosols contain substantially fewer chemicals than tobacco smoke. Thus, they are perceived as a safer alternative to tobacco smoking. However, electronic cigarette aerosols contain known carcinogens and reactive oxygen species, as well as unique constituents that might have unforeseen health consequences. We and others have reported that, in vitro and in animal models, electronic cigarette aerosols induce DNA damage and can alter gene expression reducing cellular antioxidant capacity and DNA repair. These data raise the concern that electronic cigarette use, in addition to causing DNA damage, reduces DNA repair capacity potentially increasing cancer risk associated with other genotoxics. There is a clear and urgent need to assess the genotoxic effects of electronic cigarette aerosols in the context of exclusive and dual electronic cigarette users, to protect users and bystanders, and to guide evidence-based public health policies and regulations. There have been no genotoxic studies in electronic cigarette users. Here, we propose to assess the genotoxic and non-genotoxic effects of electronic cigarette use in exclusive and dual users. Based on our preliminary data we hypothesize that: (a) vaping causes an increase in genotoxicity, which is smaller than combustible tobacco but is amplified in dual users; (b) vaping and smoking alter the oral transcriptome towards distinct oncogenic phenotypes; and (c) specific aerosol constituents determine the unique genotoxic and non-genotoxic properties of electronic cigarette mixtures. To test our hypotheses and attain our overall aims, we will pursue the following three specific aims: (1) assess the genotoxic and mutagenic effects of electronic cigarettes in exclusive and dual electronic cigarette and combustible tobacco users; (2) characterize the impact of electronic cigarette use on oral mucosa transcriptome; and (3) dissect the effect of EC aerosol constituents on cellular genotoxicity and DNA repair capacity. DNA damage, mutagenicity, and transcriptomes will be characterized in exclusive and dual electronic and combustible cigarette users. Genotoxicity will be evaluated through the targeted analysis of specific DNA adducts, but also through the use of a novel assay developed in our lab (q-PADDA) which detects with high sensitivity a broad spectrum of DNA damage lesions. The electronic cigarette aerosols particle size distribution and chemical profile will also be characterized. Functional studies will probe drivers and pathways altered by electronic cigarette use. The effects of complex EC aerosol mixtures and individual constituents on mutagenicity will be evaluated, and the mechanisms contributing to overall genotoxicity investigated. Our team is uniquely suited to use a multidisciplinary approach to dissect the oncogenic potential of electronic cigarette use. Our study will deliver critical knowledge urgently needed by clinicians to better guide their patients and by regulatory agencies to effectively regulate electronic cigarettes in a way that best serves public health.